To answer your question - yes, that’s exactly what happens. The folder gets moved to ‘Deleted Items’ and can only be seen if ‘Show all folders’ is selected in WebApp. No way to move or restore it back.

I can’t show you the folder under ‘Deleted Items’ now as I was troubleshooting the script mentioned above and it seems the folder is now lost.

Forgot to mention - This has been the case since Zarafa 7.x

UPDATE 2 - Not sure if this helps, the output of the recreate-systemfolders script when the ‘Tasks’ folder was in ‘Deleted Items’:

I would appreciate if you could provide more information on why the caldav client was able to bypass the protected system folder’s permissions before I escalate the bug/issue with Fantastical’s dev team.